1. Field of the Invention
The present invention relates to a traction control system. Such a system may be used in a vehicle having first and second driven wheels interconnected by a limited slip differential, for instance of the viscous coupled type.
2. Discussion of the Prior Art
Traction control systems are provided in vehicles, for instance driven by internal combustion engines, in order to control wheel spin. Such systems detect the presence of excessive wheel spin and reduce engine output demand, for instance by acting on an engine management system, until wheel spin falls to or below a target value. Thus, lateral instability of the vehicle caused by wheel spin can be greatly reduced or eliminated.
In a vehicle of the type in which engine power is transmitted to one or more axles with the wheels on each axle being driven by a limited slip differential, difficulties can occur when the wheels of the or one of the axles are on surfaces of substantially different frictional properties ("split .mu. surfaces"). If one wheel is on a surface of high friction and the other wheel is on a surface of low friction, the other wheel is more likely to spin. This difficulty is particularly likely to occur when trying to accelerate the vehicle on a hill. Limited slip differentials, for instance of the viscous coupled type, permit the wheels of the axle to rotate at different rates provided the difference between the rates of rotation is not excessive. If the difference becomes excessive, then the differential locks up and transmits a greater portion of engine torque to the non-spinning wheel i.e. the wheel on the higher friction surface. However, the action of the traction control system is such that, when excessive wheel spin is detected, the engine output is reduced, for instance by reducing the engine output demand signal of the engine management system. The reduction in engine output is such as to eliminate wheel spin or reduce it below a threshold level, which may be less than a very few kilometers per hour. This action thus prevents locking of the limited slip differential and transfer of torque to the wheel on the higher friction surface, so that performance of the vehicle suffers.